Green Pea galaxies: a part of diet for cosmic reionization

Discovered by citizen scientists, this galaxy type could resemble the earliest.

Six of the Green Pea galaxies found in galaxy survey data by citizen scientists. These galaxies may resemble forms in the early Universe, which produced the radiation that ionized much of the gas in the cosmos.

A major triumph of citizen science was the identification of the Green Pea galaxies by people involved with the GalaxyZoo project. Named for their unusual green color and small size, these strange objects were unknown until 2007. That's when the regular people who were helping out scientists by looking at galaxy photos started discussing the Green Peas in the project's forum.

Since then, scientists have discovered that the Green Peas are aggressive star-forming galaxies, which means they are a source of high-energy ultraviolet radiation. Unusual as they are, Green Pea galaxies could help resolve the problem of reionization of the Universe, the gradual stripping of electrons from atoms that made the cosmos transparent some 380,000 years after the Big Bang. Ultraviolet radiation is thought to have driven the reionization, but most star-forming galaxies reabsorb too much of that light to be useful for studying the process.

A new paper by A. E. Jaskot and M. S. Oey argues that galaxies much like the Green Peas could be responsible for the reionizing radiation. They analyzed the light emissions from the galaxies, and determined that their gas is thinner than in typical star-forming galaxies, which could allow more ultraviolet light into intergalactic space. The researchers also found signs in a few Green Peas of extremely massive stars, the ones most responsible for ionizing radiation.

Green Pea galaxies—commonly known as "Peas" to confuse biologists in casual conversation—are rare. GalaxyZoo volunteers only identified 251 from Sloan Digital Sky Survey (SDSS) data, out of over 1 million galaxies in the survey. Despite being relatively close to the Milky Way, the Peas are small enough that their features cannot be distinguished in the SDSS images.

While most galaxies are more red or blue in color, Peas' striking green color comes from strong oxygen emission. Specifically, this is OIII emission—that's O with the Roman numeral three. That's light from oxygen ions with two missing electrons, an uncommon source that requires extremely low gas densities.

Turning gas into stars

How do we measure how much gas ends up in new stars in a galaxy? You can measure it in total stars, but that has its issues. Most stars are much less massive than the Sun, but a few are absolute monsters. The better measure of star formation, then, is not how many new stars are born, but how much total gas is converted into stars. Peas form, on average, the equivalent of 13 new Suns every year, while the Milky Way (typically for a middle-aged spiral galaxy) make the equivalent of about 4 new Suns annually. However, the Milky Way is over 100 times the mass of an average Pea galaxy, meaning the smaller objects are much more aggressive in their star formation.

In addition to OIII emission, several Peas exhibited ionized helium (HeII) and hydrogen (HII), hallmarks of ultraviolet emission. In some galaxies, ionization of that sort is due to the supermassive black hole at the galaxy's center, but the Peas don't bear signs of active black hole behavior. Another potential source is massive stars, many times brighter than the Sun, which emit a lot of ultraviolet light. Still another option is very hot gas, heated to incandescence by supernova explosions and outflows of particles from big stars.

With the absence of supermassive black hole restlessness, the authors of the study settled on large stars and star formation as the most likely ionizers.

The next piece of the puzzle was the apparent thinness of gas in the Peas. That's hard to measure directly for such small galaxies. But the researchers compared the ultraviolet emission and chemical signatures (as gleaned from the galaxies' spectra) to those of galaxies similar to Peas, and found that the Peas were relatively transparent to radiation. That means the Peas themselves are likely to allow ultraviolet light from internal sources to escape from the galaxies.

For light such as this to be responsible for reionization, something very like Green Pea galaxies would have to exist at much earlier eras. Thankfully, astronomers have observed galaxies with similar chemical and ionization signatures at greater distances, though again their small size works against a full description. However, by comparing Peas to other galaxies in the nearby Universe, then extending that comparison to distant galaxies, this study may have brought us closer to solving the mystery of reionization.

I think your confusing recombination and reionization timeframes. Recombination (forming neutral atoms) happened at about "379,000 years after the Big Bang." Reionization happened "between 150 million and one billion years after the Big Bang"

The initial reionization can't be caused by green peas because their color is from multiply ionized oxygen which did not yet exist.

As jlpicard2 points out, reionization happened later than recombination. And the stars in the first generation, Population III (hypothesized), were extremely short-lived, probably only lasting a very few million years before exploding. As long as their masses were not too large or small, those supernovae would have seeded oxygen and other metals into the interstellar medium. So there is plenty of time for the oxygen to have been created.

I have a naive question about the stars in these galaxies. I understand that stars radiate in very broad spectra including ultraviolet, but there is a peak somewhere that gives the star a characteristic color. Does the Main Sequence actually extend into the infrared and/or ultraviolet? I can easily see the boundary between brown dwarf and true star keeping things out of the infrared, but at the other end of the spectrum it seems intuitive that the "bluest" stars don't care much what wavelengths we can see with our eyes.

An "ultraviolet" star would look to us like a regular bluish star, but dimmer than expected from its mass and energy.

Even if the Main Sequence is somehow constrained to visible light, those constraints may have been relaxed for really early stars in really early proto-galaxies.

I have a naive question about the stars in these galaxies. I understand that stars radiate in very broad spectra including ultraviolet, but there is a peak somewhere that gives the star a characteristic color. Does the Main Sequence actually extend into the infrared and/or ultraviolet? I can easily see the boundary between brown dwarf and true star keeping things out of the infrared, but at the other end of the spectrum it seems intuitive that the "bluest" stars don't care much what wavelengths we can see with our eyes.

An "ultraviolet" star would look to us like a regular bluish star, but dimmer than expected from its mass and energy.

Even if the Main Sequence is somehow constrained to visible light, those constraints may have been relaxed for really early stars in really early proto-galaxies.

Here is my simplified understanding of it:

The Main Sequence is simply an apparent grouping on a Hertzsprung-Russell diagram that appears when you plot the most common type of stars, dwarf stars, during the period of their lives when they fuse hydrogen in their core. "Visible" light has nothing to do with it; the top and low end of the Main Sequence just correspond approximately to the largest mass and lowest mass dwarf stars, respectively. If a star is smaller than the low end of the red dwarfs, it isn't big enough to perform fusion, so it is a brown dwarf, which just seeps out infrared, slowly losing the heat that came from the collapse that formed it. At the high mass end of the dwarfs, the star's lifetime becomes shorter and shorter because of the higher rate of fusion, and so there are fewer and fewer of them as you go up in mass- most of them have all already become supergiant etc. stars, on a separate section of the HR diagram. Although I think there are absolute limits on stellar size- the star has to be in thermal equilibrium, and if it is too big, it will pulsate and throw off mass until it is small enough to remain stable, which I think determines the limit at the high end of the main sequence.

All of these stars (except brown dwarfs) emit light across the electromagnetic spectrum, both far below and far above visible light. The ends of the categories aren't determined by what we see, but by the physics that determines the range of sizes (and compositions) that a star can be stable without transitioning to a different form of power (hydrogen fusion in the core, hydrogen fusion outside the core, helium fusion, etc).

For example, our star is a yellow dwarf, and is on the main sequence, but will leave it when the core runs out of hydrogen to fuse, and then it will swell into a red giant, where the fusion takes place outside the core. Once that process finishes, it starts fusing helium to carbon for awhile and then runs out and becomes a white dwarf, which doesn't do any fusion but still shines with remnant heat, which dissipates until it becomes a cold black dwarf, but this cooling process takes much longer than the life of the universe so far, so there aren't any cold black dwarfs yet.

The Main Sequence is simply an apparent grouping on a Hertzsprung-Russell diagram that appears when you plot the most common type of stars, dwarf stars, during the period of their lives when they fuse hydrogen in their core. "Visible" light has nothing to do with it; the top and low end of the Main Sequence just correspond approximately to the largest mass and lowest mass dwarf stars, respectively.

So for Green Pea galaxies to have had "ultraviolet" stars, the conditions would have had to have been different to allow a larger maximum mass. This is unlikely in the current generation of Green Peas, but might have been true during reionization when these Oompa Loompa galaxies were doing their most important work.

The Main Sequence is simply an apparent grouping on a Hertzsprung-Russell diagram that appears when you plot the most common type of stars, dwarf stars, during the period of their lives when they fuse hydrogen in their core. "Visible" light has nothing to do with it; the top and low end of the Main Sequence just correspond approximately to the largest mass and lowest mass dwarf stars, respectively.

So for Green Pea galaxies to have had "ultraviolet" stars, the conditions would have had to have been different to allow a larger maximum mass. This is unlikely in the current generation of Green Peas, but might have been true during reionization when these Oompa Loompa galaxies were doing their most important work.

Yes, I think a leading theory about the first "Population III" stars is that because of their low metallicity and the warmer interstellar medium, they could have been much larger than stars can be now, up to several hundred solar masses. http://en.wikipedia.org/wiki/Population ... _III_stars

So for Green Pea galaxies to have had "ultraviolet" stars, the conditions would have had to have been different to allow a larger maximum mass. This is unlikely in the current generation of Green Peas, but might have been true during reionization when these Oompa Loompa galaxies were doing their most important work.

While it is theorized that there were much larger stars in the past, it's perfectly possible for a star today to be an "ultraviolet" star, as in its peak brightness is in the UV part of the spectrum.

For example Zeta Puppis is a blue supergiant 22 times more massive than the sun with an O-type spectrum at a surface temperature of 42,000K.

At that temperature, 97% of its emissions are in the UV range. Wolfram Alpha link:

So for Green Pea galaxies to have had "ultraviolet" stars, the conditions would have had to have been different to allow a larger maximum mass. This is unlikely in the current generation of Green Peas, but might have been true during reionization when these Oompa Loompa galaxies were doing their most important work.

While it is theorized that there were much larger stars in the past, it's perfectly possible for a star today to be an "ultraviolet" star, as in its peak brightness is in the UV part of the spectrum.

For example Zeta Puppis is a blue supergiant 22 times more massive than the sun with an O-type spectrum at a surface temperature of 42,000K.

At that temperature, 97% of its emissions are in the UV range. Wolfram Alpha link:

While it is theorized that there were much larger stars in the past, it's perfectly possible for a star today to be an "ultraviolet" star, as in its peak brightness is in the UV part of the spectrum.

For example Zeta Puppis is a blue supergiant 22 times more massive than the sun with an O-type spectrum at a surface temperature of 42,000K.

At that temperature, 97% of its emissions are in the UV range.

[The surface temperature is the characteristic peak temperature of a fitted black body spectrum, which as a thermal radiator the star surface fits rather well, modulo absorption lines and what not.]

On the other end, some M stars (red dwarfs) emit mostly into IR. So much so that it was very helpful with recent finds of pigments for oxygenating photosynthesis that works in near IR. (Anoxic photosynthesis prefer IR, so no problem there.) [ http://en.wikipedia.org/wiki/Chlorophyll_f ]

Meaning that the smallest stars can still be habitable for complex multicellular oxygen using organisms, everything else alike. (If anyone wants to raise the tedious scepter of tidal lock, I preempt with Venus which thick atmosphere distributes heat without tipping landers over in too strong winds.)

Thanks for the replies, so it looks like the Main Sequence does actually poke beyond the visible (to humans) portion of the spectrum. My thought was that Green Peas would have more UV stars than a typical modern galaxy, though it may be sufficient simply to have less dust absorbing the UV from a more familiar distribution of stars.